Driver and receiver circuits for a remotely arranged circuit are sufficiently known and serve for example for supplying voltage to remotely arranged sensors and for receiving sensor signals from such sensors. One example of such a driver and receiver circuit is the integrated component TLE7719-T from Infineon Technologies AG, Munich, which serves for supplying voltage to airbag sensors and for communication with airbag sensors. This component is described in the data sheet “SatRIC™ Airbag Satellite Receive IC, TLE7719-T”, Version 1.1, 2004 Sep. 13, Infineon Technologies AG, Munich.
In order to afford a better understanding, the basic construction of such a driver and receiver circuit and its interconnection with a remotely arranged circuit, which is referred to hereinafter as “satellite circuit”, are explained with reference to FIG. 1.
In FIG. 1, the driver and receiver circuit is designated by the reference symbol 10 and the satellite circuit is designated by the reference symbol 20. The driver and receiver circuit 10 has output terminals 101, 102 for connecting a line connection 30. Via said line connection 30, during operation, voltage is supplied to the satellite circuit 20 and information is transmitted from the satellite circuit 20 to the driver and receiver circuit 10.
The driver and receiver circuit 10 makes a supply voltage Vbias available via the connecting terminals 101, 102 at the line connection 30 by means of a voltage source 13. By means of a current measuring arrangement 12, the driver and receiver circuit 10 additionally determines a current Iout flowing via the terminals 101, 102 and supplies an item of current information to a central control circuit 14.
The satellite circuit 20 is connected to the line connection 30 via terminals 201, 202 and is designed to modulate the current flowing via the line connection 30 such that it assumes a first or a second current value. For this purpose, the satellite circuit 20 has two current sources 21, 22, for example, a first 21 of which is always connected to the line connection 30 and a second of which can be connected in parallel with the first current source 21 via a switch 23 according to a control signal 24. The satellite circuit 20 comprises a sensor 26, for example, such as an airbag sensor, for example, which provides a sensor signal S26, according to which a driver circuit 24 generates the control signal S24 for the modulation of the current Iout.
An internal supply voltage Vint of the satellite circuit 20 is generated from the voltage Vbias present at the line connection 30 by a voltage supply circuit 25.
FIG. 2 shows by way of example the temporal profile of the current Iout flowing via the line connection 30. During disturbance-free operation, said current assumes two current values, a first current value I1 corresponding to the current supplied by the first current source 21, and a second current value I2 corresponding to the sum of the currents supplied by the two current sources 21, 22 of the satellite circuit 20. The current Iout flowing via the line connection 30 represents a coded or modulated signal. In this case, the information to be transmitted from the satellite circuit 20 to the receiver circuit 10 may be contained in the two different levels of the current signal Iout or in the level changes of said current signal Iout. The information from the satellite circuit 20 to the receiver circuit 10 is transmitted in the form of “data packets”, for example, the individual information symbols of which are transmitted successively within a time duration Td.
The receiver circuit 10 is designed to demodulate and, if appropriate, decode the information contained in the current signal Iout and to generate a control signal S10, which may be for example a firing signal for an occupant protection system of a motor vehicle.
An erroneous operating state can occur in the case of such a receiver circuit when a short circuit occurs between the connecting terminals 101, 102, as is illustrated by the reference symbol 42 in FIG. 1. A further erroneous operating state is present when there is a short circuit between the line connection 30 and a terminal for a supply potential Vs. The effects of such an error are that the communication link to the satellite circuit 20 is interrupted. Furthermore, the driver and receiver circuit 10 may be damaged on account of such an error because the currents flowing during such an error state may be considerably greater than the currents flowing during normal operation. Moreover, a permanent overcurrent may lead to a dip in the voltage supply and thus also to a disturbance of adjacent channels (not illustrated).
In order to protect the driver and receiver circuit 10 against such high currents, it is known, in principle, to provide a current limiting arrangement that limits the output currents of the circuit 10. However, since the receiver circuit 10 must be able to generate comparatively high currents for short time durations, the current limit value has to be set comparatively high, so that in the event of permanently flowing currents corresponding to the current limit value, the circuit 10 may be overheated and possibly damaged.
An over-temperature shutdown may protect against such overheating, although devices that provide such shutdown generally respond only in a time-delayed manner depending on the ambient temperature.
There is a need, therefore, for a driver and receiver circuit which is better protected against error states.